Normal pressure hydrocephalus

Normal-pressure hydrocephalus (NPH), also called communicating hydrocephalus and malresorptive hydrocephalus, is a condition in which excess cerebrospinal fluid (CSF) occurs in the ventricles, and with normal or slightly elevated cerebrospinal fluid pressure. As the fluid builds up, it causes the ventricles to enlarge and the pressure inside the head to increase, compressing surrounding brain tissue and leading to neurological complications. The disease presents in a classic triad of symptoms, which are urinary incontinence, dementia, and gait disturbance. The disease was first described by Hakim and Adams in 1965.[1]

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NPH exhibits a classic triad of clinical findings (known as the Adams triad or Hakim's triad). The triad consists of gait disturbance, dementia, and urinary incontinence (commonly referred to as "wet, wacky, and wobbly" or "weird walking water").

Gait disturbance is present in nearly all patients and usually the first symptom. This is caused by expansion of the lateral ventricles to impinge on the corticospinal tract motor fibers. The typical gait abnormality in NPH is a broad-based, slow, short-stepped, "stuck to the floor", or "magnetic" movement. The gait abnormalities in NPH may bear resemblance to a gait associated with Parkinson's disease. The gait disturbance can be classified as mild, marked, or severe. Marked is classified as difficulty walking because of considerable instability. Severe is classified as walking without aids (such as cane or wheeled walker) is not possible.[2][3]

Dementia presents as progressive cognitive impairment which is present in 60% of patients at time of treatment. This is caused by distortions predominantly at the frontal lobe and the subcortex.[4] Initial deficits involve planning, organization, attention, and concentration. Further deficits include difficulty managing finances, taking medications, driving, keeping track of appoints, daytime sleeping, short-term memory impairments, and psychomotor slowing. Late stage features include apathy, reduced drive, slowed thinking, and reduced speech.

Every day, the body makes roughly 600-700 ml of CSF, and about the same amount is reabsorbed into the bloodstream. Hydrocephalus is due to an imbalance between the amount of fluid produced and its absorption rate. Enlarged ventricles put increased pressure on the adjacent cortical tissue and cause myriad effects in the patient, including distortion of the fibers in the corona radiata. This leads to an increase in intracranial pressure (ICP). The ICP gradually falls, but still remains slightly elevated, and the CSF pressure reaches a high normal level of 150 to 200 mm H2O. Measurements of ICP, therefore, are not usually elevated. Because of this, patients do not exhibit the classic signs that accompany increased intracranial pressure such as headache, nausea, vomiting, or altered consciousness, although some studies have shown pressure elevations to occur intermittently.

The exact pathogenesis is unknown, but consensus on some mechanisms include:

An imbalance exists between production and resorption of CSF.

The resistance to CSF outflow is often elevated.

The disease is not caused by overproduction of CSF or obstruction of CSF flow at the ventricles.

The syndrome is often divided into two groups, primary (also called idiopathic) and secondary, based on cause. The underlying etiology of primary NPH has not yet been identified. Primary NPH affects adults age 40 years or older, most commonly affecting the elderly. Secondary NPH can affect persons of any age and occurs due to conditions such as subarachnoid hemorrhage, meningitis, brain surgery, brain radiation, or traumatic brain injury.

Evan's index is the ratio of maximum width of the frontal horns to the maximum width of the inner table of the cranium. An Evan's index more than 0.31 indicates hydrocephalus.[5]

Imaging of NPH (CT at left) compared to cerebral atrophy (MRI at right), both at the level of the posterior commissure: in the left image, the CSF spaces over the convexity near the vertex are narrowed ("tight convexity", red circle), as are the medial cisterns (red circle) - these are typical findings of NPH. On the right image, however, the CSF spaces over the convexity near the vertex (red arrow) and the medial cisterns (green arrow) are widened, a finding consistent with brain atrophy. The blue lines in both images indicate the callosal angle: an angle less than 90° is typical of NPH (left image), while an angle greater than 90° is typical of brain atrophy (right image). The blue arrows indicate periventricular signal alterations. The unilateral occurrence of these alterations (right image) suggests they are probably due to vascular encephalopathy. The abnormalities seen in the left image may well represent transependymal CSF diapedesis due to NPH.[6]

MRI scans are preferred. The distinction between normal and enlarged ventricular size by cerebral atrophy is difficult to ascertain. Up to 80% of cases are unrecognized and untreated due to difficulty of diagnosis.[7] Imaging should also reveal the absence of any cerebral mass lesions or any signs of obstructions. Although all patients with NPH have enlarged ventricles, not all elderly patients with enlarged ventricles have primary NPH. Cerebral atrophy can cause enlarged ventricles, as well, and is referred to as hydrocephalus ex vacou.

The Miller Fisher test involves a high-volume lumbar puncture (LP) with removal of 30-50 ml of CSF. Gait and cognitive function are typically tested just before and within 2-3 hours after the LP to assess for signs of symptomatic improvement. The CSF infusion test and the lumbar test are similar tests to Miller Fisher test. The tests have a positive predictive value over 90%, but a negative predictive value less than 50%. The LP should show normal or mildly elevated CSF pressure. CSF should have normal cell contents, glucose levels, and protein levels.[8][9][10]

For suspected cases of NPH, CSF shunting is the first-line treatment. The most common type used to treat NPH is ventriculoperitoneal (VP) shunts, which drain CSF fluid to the peritoneal cavity. Adjustable valves allow fine-tuning of CSF drainage. NPH symptoms reportedly improve in 70-90% of patients with CSF shunt. Risk-benefit analyses have shown beyond any doubt that surgery for NPH is far better than conservative treatment or the natural course.[11]

Gait symptoms improve in ≥ 85% patients. Cognitive symptoms improve in up to 80% of patients when surgery performed early in disease course. Incontinence improves in up to 80% of patients, but only in ≤ 50%-60% of patients with shunt implanted late in disease course. The most likely patients to show improvement are those who show only gait disturbance, mild or no incontinence, and mild dementia. The risk of adverse events related to shunt placement is 11%, including shunt failure, infections such as ventriculitis, shunt obstruction, over- or under-drainage, and development of a subdural hematoma.[12][13][14]

The majority of cases are primary NPH. The incidence of NPH increases with advancing age, and most patients are over the age of 60. Its prevalence is reported to be less than 1% in persons under the age of 65, and up to 3% for persons aged 65 or older. No difference in incidence is seen between men and women.[4][15][16] Among individuals with dementia, the incidence of NPH is thought to be between 2 and 6%.